This invention relates to the oligomerization of alkenes, preferably propylene to linear and slightly branched olefins by employing novel catalysts comprising physical mixtures of cobalt oxide on carbon with certain catalyst support materials such as refractory oxides and molecular sieves. In another embodiment, the invention describes a method of producing said novel catalysts by either of the following two processes, namely, (1) impregnating activated carbon with a solution containing a cobalt amine complex, as hereinafter described, followed by decomposing the cobalt salt to cobalt oxide; or, (2) impregnating carbon with cobalt nitrate, adding aqueous sodium hydroxide or potassium hydroxide at room temperature to convert the cobalt nitrate to cobalt hydroxide, removing the sodium or potassium nitrate with water extraction and decomposing cobalt hydroxide to cobalt oxide at high temperatures. The so-formed cobalt on carbon is thereafter combined with a catalyst support material comprising refractory oxides and/or molecular sieves in order to form the active catalyst system of the instant invention. The use of the novel catalyst compositions in the process described herein permits the polymerization of alkenes such as propylene at mild conditions to yield olefin oligomers rich in linear and slightly branched olefins such that significant amounts of desirable trimers and tetramers are produced. In addition, the use of these novel catalysts in the oligomerization reaction provides a means whereby a broad range of activated carbons such as, for example, petroleum-base activated carbons, can be employed. Moreover, it has been found that the product distribution can be varied by varying the catalyst support.
It is known that cobalt oxide on carbon is a good catalyst for the dimerization of propylene to produce high yields of linear hexenes. The difficulty with employing cobalt oxide on carbon as a catalyst in the prior processes was that in order to have an active catalyst produced, the cobalt oxide had to be prepared by decomposing cobalt nitrate at high temperatures. This decomposition of cobalt nitrate impregnated on the carbon support is hazardous because of the potentially explosive nature of such a mixture. Additionally, other difficulties in the prior art techniques reside in the fact that cobalt oxide on carbon is essentially a dimerization catalyst and therefore does not produce significant amounts of desirable trimers and tetramers.
Typical of the prior art processes and catalysts for olefin oligomerization may be found described in British Pat. No. 1,102,298. This patent teaches the addition of an alkali metal as the promoter to the cobalt activated carbon catalyst; said promoter is said to increase the activity of the catalysts toward olefin oligomerization. The catalysts employed by the patentees are those prepared from cobalt salts and essentially cobalt nitrate is the preferred cobalt salt seen necessary by the patentees to provide an active enough catalyst for olefin oligomerization.
In another British Pat. No. 1,124,766, the preparation of oligomerization catalysts is described which comprises the steps of depositing a nitrate and a cobalt compound on activated carbon, heating the carbon on which the cobalt compound and the nitrate have been deposited until all the nitrate has reacted and heating the carbon in an inert atmosphere at temperatures in the range of 200.degree.-600.degree. C. The patentees of this reference state that it is an essential feature of their invention that at least a part of the total nitrate content of the carbon, prior to the nitrate reaction step being carried out, be supplied by nitrates other than cobalt nitrates and suggest the use of transition metal nitrates for this purpose.
Another prior art process for the preparation of catalysts suitable for olefin oligomerization is described in British Pat. No. 1,151,266. This patent discloses an improved method of making carbon/cobalt catalysts that does not include the necessity of using cobalt nitrate as a source of the cobalt. According to the invention described by these patentees, the process for the production of cobalt/carbon catalyst comprises heating activated carbon onto which cobalt, copper and/or nickel have been introduced by deposition of their salts, other than the nitrates, in a mixture of the inert gas and molecular oxygen at temperatures in the range of 150.degree.-450.degree. C. and thereafter in an inert gas containing no molecular oxygen. The patentees further state that it is preferred to avoid the use of salts which contain sulfate, phosphate or other anions which will leave harmful residues on the catalysts. Suitable salts that may be employed according to this patent are acetates, propionates, butyrates, oxalates, naphthanates nonanoates. The salts of copper, nickel and cobalt in this process are conveniently deposited on the carbon by impregnation of the carbon with solutions of the salt. The catalyst produced in the aforementioned method are useful for the production of hexenes from propylene or octenes from butenes. The catalysts in general provide a method for converting ethylene, propylene and butenes in high yields of straight chain C.sub.5 to C.sub.8 olefins without the production of appreciable amounts of olefins containing more than 8 carbon atoms. Essentially, then, these catalysts are predominantly dimerization catalysts.
In still another prior art process, the production of a catalyst suitable for olefin oligomerization comprises depositing a cobalt compound capable of yielding cobalt or an oxide of cobalt on heating under the catalyst preparation conditions on activated carbon to form a cobalt/carbon complex; oxidizing the cobalt/carbon complex with molecular oxygen with temperatures in the range of 100.degree.-150.degree. C., and thereafter activating the oxidized cobalt/carbon complex by heating it in an inert atmosphere at temperatures in the range of 150.degree.-550.degree. C. The patentees of this invention state that the process may be carried out more safely than processes using cobalt nitrate as a starting material and suggest that useful salts of cobalt include cobalt formate, cobalt acetate and cobalt isooctoate. This process also discusses that the heating of the cobalt/carbon complex in an inert atmosphere is essential to give catalytic activity to the cobalt/carbon complex.
Also, in British Pat. No. 1,216,272, the use of nickel exchanged X-type molecular sieve zeolites is cited to be a useful catalyst for the dimerization of olefins. The catalyst is prepared by exchanging an X-type molecular sieve zeolite with a nickel salt in an ionic solution of sufficient strength to give substantially complete ion exchange with the molecular sieve, washing the catalyst with an undissociated or a weakly dissociated solvent for the nickel salt until substantially all the excess nickel salt is removed, and finally activating the catalyst by heating in an inert gas between 200.degree. and 800.degree. C. Preferably, according to the patentees of this invention, the exchange is carried by refluxing the molecular sieve with the nickel salt solution.
However, all of the prior art processes discussed above do not provide for a catalyst which produces olefin oligomers rich in linear and slightly branched olefins without the attendant difficulties as described in said prior art processes. It has been found by applicant that an inactive catalyst support in combination with an essentially inactive cobalt oxide on carbon provides an active catalyst system which results in a catalyst capable of oligomerizing alkenes such as propylene at mild conditions to yield olefin oligomers rich in linear and slightly branched olefins containing significant amounts of desirable trimers and tetramers.
By "inactive" is meant that the catalyst supports, namely refractory oxides and molecular sieves, used in this invention do not oligomerize olefins to any extent at the temperatures used in the invention. If any oligomerization should occur, the oligomers would be highly branched. The cobalt oxide on carbon catalysts are likewise "inactive" in that catalysts evidence slow dimerization rates and particularly slow rates in the formation of trimers and tetramers. Physical mixtures of cobalt oxide on carbon and such catalyst supports, however, have been unexpectedly found to increase the rate of formation of dimers, as well as trimers and tetramers. As mentioned above, cobalt oxide on carbon is essentially a dimerization catalyst and the rate of dimerization is a function of the molecular weight of the olefin. That is to say, the higher the molecular weight of the olefin, the slower the rate of dimerization. However, the present invention substantially overcomes this problem in that, when using physical mixtures of the catalyst as per the invention described herein, the rate of dimerization is increased, even for higher molecular weight olefins. While applicant does not wish to be bound by any theory, it is believed that tetramer production from propylene is possible with the catalyst system described by means of the present invention because hexenes are dimerized at an appreciable rate. The trimer of propylene is, therefore, believed to be formed from hexenes and propylene.